Corneal collagen crosslinking (CCXL) has been used since the late 1990s to treat keratoconus. Recently it has been applied to healthy eyes to affect refractive changes in the shape of the cornea, and to eyes with certain antibiotic resistive infections.
The standards of corneal collagen crosslinking for treatment of keratoconus set the dose at an energy per area of 3 mW/cm2, delivered uniformly over the center 8 mm of the cornea for 30 minutes. This sums to about 5.4 J/cm2 of energy, or a total of about 2.7 J. However, the goals of CCXL for refractive correction (and anti-infective use) are different than that of keratoconus treatment, and thus the dosimetry, typically differ from this standard protocol.
CCXL for vision correction achieves refractive shape change of the cornea by creating chemical bonds between the protein layers in the corneal stroma. These bonds (crosslinks) increase the stiffness of the cornea in the region crosslinked. This increased stiffness changes the balance between the cornea tension and the intraocular pressure. Through mechanisms not completely understand in the industry, within a few days of CCXL therapy, physiologic processes reshape the cornea. The amount of reshaping, and thus the degree of curvature correction, is determined by a number of treatment parameters, including the amount and rate of energy delivery, the treatment time, and the shape and size of the treated area on the cornea. For myopia (nearsightedness), the center of the cornea is stiffened; for hyperopia (farsightedness), an annulus around the periphery of the cornea is stiffened. For more complicated corrections such as astigmatism, custom patterns are used.
Currently, there exists a fixed optical mask that is placed on the cornea in the form of an etched contact lens patterned to spatially modulate the UV dose delivered across the cornea. However, by this method, a custom-etched contact lens would have to be provided for each individualized patient prescription.